The present invention generally relates to gas turbine engine's shrouded blades, and more particularly, to substantially planar ribs used with gas turbine engine shrouded blades.
Gas turbine blades are rotating airfoil shaped components in series of stages designed to convert thermal energy from a combustor into mechanical work of turning a rotor. Performance of a turbine can be enhanced by sealing the outer edge of the blade tip to prevent combustion gases from detouring from the flowpath to the gaps between the blade tip and the inner casing. A common manner for sealing the gap between the turbine blade tips and the turbine casing is through blade tip shrouds. Accordingly, a typical turbine blade has: (i) a root that adapts the blade to be secured in an interlocking manner to a rotatable disc so that the blade extends radially outwardly from the disc, (ii) a platform adjacent to the root, (iii) a shroud radially spaced apart from the platform and (iv) an airfoil extending between the platform and the shroud. For reference purposes, the shroud located at a tip of the airfoil is said to be radially outward of the airfoil (and thus is often called an outer shroud), while the root is radially inward of the airfoil.
Not only do shrouds enhance turbine performance by reducing tip leakage loss, but they serve as a vibration preventing element. The shroud acts as a mechanism to change the blade natural frequency and in turn minimizes failures due to extended resonance time of the blade at a natural frequency.
While providing airfoils with outer shrouds contributes to general improvement of the turbine blades in numerous applications, it inevitably increases the centrifugal load on the turbine, thereby causing higher stresses in the airfoil. During operation, the turbine blades spin on a disc, about the engine axis. An exemplary typical industrial application includes a disc speeding at 3,600 revolutions per minute (rpm), while in gas turbine engines used in aircraft field the turbine rotation speed can reach even above 14,000 rpm. The heavier the blade, the more load and stresses are found on the interface between the blade root and the disc slot, for a given rotation speed. Thus, the excessive loading on the blade root and the disc resulting from the presence of the outer shroud can reduce the overall life of each component.
Another drawback to shrouds is creep curling of the blade shrouds. Depending on the thickness of the shroud, the shroud edges can curl up at their ends and introduce severe bending stresses in the fillets between the shroud and blade tip. Shrouds curl due to the bending load on the edges of the shroud from gas pressure loads as well as centrifugal loads. While a known way to improve the shroud resistance to curling is to increase the section thickness of the shroud uniformly, thereby producing a stiffer shroud, this involves additional weight being added to the shroud.
Yet further, the tangential extension from the airfoil supporting such an outer shroud may generate a bending stress at the intersection of the airfoil and of the shroud. In order to reduce the stress concentration at the airfoil at the shroud intersection, fillets of variable radius have been used. However, such fillets may result in a reduction of the flow area.
Providing gas turbine engine shrouded blades with acceptable levels of structural properties, including the ability to withstand imposed centrifugal loads and to maintain sealing capabilities, remains the area of constant interest and development. Some existing systems have various shortcomings relative to certain applications.
For example, US 2016/0032733 A1 discloses a turbine blade comprising an airfoil provided at its tip with an outer shroud and at least one gusset defined by a plateau projecting in a radial inward direction from the main inner surface of the shroud, i.e. the surface of the shroud being radially inward, facing the toward an axis of the turbine.
Further, U.S. Pat. No. 5,971,710 discloses a turbine blade for a gas turbine engine, said blade including a permanent machining datum extending radially from a pocket in the outer shroud of the blade. The datum is spaced from a sidewall of the pocket so that the datum is peripherally continuous irrespective of whether the blade is in a prefinished state or in a completely finished state. Since the datum's peripheral continuity survives the original manufacturing process, the datum is available for use in post-manufacturing inspection and repair operations.
Yet further, EP 1 451 446 B1 discloses a gas turbine blade having a blade tip shroud with tapered shroud pockets to remove excess weight from the shroud while not compromising shroud bending stresses. The resulting rib created between the tapered pockets provides a surface, away from possible areas of stress concentration, for drilling radial cooling holes. The rib also slightly extends beyond the knife edge but does reach the aft edge of the outer shroud.
US 2015/0017003 A1 discloses a gas turbine engine blade provided at its radially outer portion with a shroud. At its outer surface the shroud is provided with a thickened stiffener placed in a central portion of the shroud and elevated in the outwardly radial direction from this surface. The height of the thickened stiffener decreases to the circumferential sides of the outer shroud. Apart from a single specific embodiment, wherein the stiffener has cross-section of a circular segment, no preference is given to any specific geometry and/or relative dimensions of the shroud and/or the stiffener.
An exemplary method of producing a blade is casting process. Casting is a manufacturing process by which a liquid material—including in particular a metal/a metallic alloy in a liquid phase—is usually poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to solidify. Customary methods require expensive machining, such as grinding of the outer part of the shroud platform to keep tight tolerances required in such high-performance elements like turbine blades.
Accordingly, there is a constant need to further improve outer shroud configuration, especially aimed at reduction of overall blade mass, which in turn would reduce the amount of pull on the turbine disc, increasing the life of both the turbine blade root and corresponding disc locations. Further, such improved shroud configuration should preferably have positive impact on shroud platform vibrational properties and at the same time provide sufficient stiffening of the shroud, so that it could withstand normal operation conditions with little or no creep curling of the shroud edges or bending stress at the intersection of the airfoil and of the shroud. Yet further, no change of the shroud configuration should lead to undesired reduction of the flow area. Finally, reducing or preferably eliminating the need for expensive machining of the outer side of the shroud platform would greatly reduce the overall costs and time of blade production.
As it is well known to those skilled in the art, any slight modification of the configuration of the shroud elements may have significant impact on the natural frequency of each single blade, or—once the entire set of blades of the turbine is heated up during operation—the natural frequency of the entire turbine. Due to extremely high complexity of mechanical phenomena occurring during the turbine blades performance and grate number of factors to be taken into account (such as blade material characteristics, flow characteristics, thermal behaviour, operating parameters of the turbine . . . ) it is impossible to anticipate without extensive study and complex calculations whether a given configuration of a shroud would bring any improvement in at least one of the areas mentioned above or vice versa. Therefore, the development of a new shroud design is costly and requires highly qualified engineers and access to expensive analytic programs, that perform complex numerical analysis, not to the mention a great amount of time needed for the overall process, often including further experimental verification.